[B]Greetings David; Yes the length of string does equal the focal length of the lens: I hold the knot on my forehead above my viewing eye and find the area in the frame to be near enough. Linhof, and others made an optical Zoom finder with the field of view frames marked on the glass, these are sold on Ebay and elsewhere but the cut out frame does the job for me.
BTW. you can work out a smaller cut-out in the sheet film proportions that is smaller and then you have shorter string lenghts and a smaller card to carry round. BarrieB.